ASCAT winds, Tropical Airmass RGB, True Colour with High-Resolution Night-time IR RGB, Cloud Phase RGB, Cloud Type RGB
In April 2023, Ilsa was the sixth named tropical cyclone of the 2023 season & the fifth severe cyclone to hit Australia.
03 May 2023
02 May 2023
By Djordje Gencic, Ivan Smiljanic, Jochen Kerkmann and Carla Barroso
The cyclone reached the highest category 5 on the Australian BOM tropical cyclone scale. It made landfall in northwestern Australia, close to Port Hedland, on 13 April, with sustained winds of 205km/h and gusts up to 235km/h, causing widespread damage in the region.
Tropical Cyclone Ilsa started as a tropical depression forming in the Arafura Sea in Indonesia on 5 April. Over the subsequent two days the system strengthened, while gradually moving towards the south west. On 7 April, the Joint Typhoon Warning Center issued the first Tropical Cyclone Alert, as the system became a tropical cyclone.
On 10 April the Metop-B OSI SAF ASCAT product estimated 10m wind speeds up to 21m/s (Figure 1).
By late 11 April the system had strengthened further to Category 1 and was named Ilsa.
Throughout 12 and 13 April, as the system moved southwestward, it further intensified, reaching category 5 equivalent tropical cyclone on 13 April, just north of the coast of northwestern Australia.
The highest ten-minute sustained winds reached 218km/h at Bedout Island, beating the previous record of cyclone George in 2007.
Ilsa made landfall at 16:00 UTC on 13 April, approximately 120km northeast of Port Hedland. As the storm moved further inland it quickly weakened, and had dissipated by 16 April.
Two fishing boats were caught up in the storm, and the crew of one of them is missing. The damage is estimated to be four million AUD.
The cyclone's development can be seen the loop of Tropical Airmass RGBs for the whole lifetime span (Figure 2).
Figure 3 shows the loop of True Colour RGB (which is similar to what the human eye would see) during day time and high resolution IR combination overnight. This composite will soon be available with new MTG FCI data.
The total path of Ilsa can be seen in Figure 4, with associated categorisation of the cyclone.
The state of the cyclone a few hours before landfall is shown in the comparison of Sentinel-3 SLSTR Cloud Phase RGB and Cloud Type RGB (Figure 5).
With FCI data being in commissioning phase it is always good to look at these composites, which will be a standard going forward. Both of them provide valuable information on cloud type and cloud particle phase. In the Cloud Phase RGB the scene is dominated by Ilsa's cloud tops, which are represented as dark and light blue, meaning the cloud tops consist purely of ice particles, where dark blue means large ice crystals, and light blue to cyan usually means small ice particles, which can be expected to originate from strong thunderstorms inside the tropical cyclone.
A look at Cloud Type RGB at the same location shows bright yellow, which confirms these are thick cirrus (high level) clouds. Towards the edges of the cyclone, it can be seen in the Cloud Phase RGB that the clouds are thinning out and are a more transparent blue, suggesting this is still purely ice cloud but with lesser optical thickness. The cloud thickness can also be inferred from the Cloud Type RGB where the bright yellow is replaced by orange, red and dark red, implying overall lower input from both the red (1.3µm) and, especially, green (0.6µm) components, again suggesting the clouds are thinning out towards the edges of the cyclone.
It should also be noted that in the Cloud Phase RGB, the convective bands east of the cyclone nicely display the vertical phase structure of the cloud, where the lowest parts are bright pink, suggesting a liquid phase. Moving up towards the anvil, the colours change to violet, dark blue, and, at the very top, light blue, pointing to phase changes into large ice particles, and, finally, small ice particles. This is due to both a slightly decreasing red component (1.6µm — related to cloud phase — low for ice, high for water), as well as an increase of the green component (2.25µm — related to cloud particle size — low for large particles and high for small particles). The same clouds in the Cloud Type RGB are shown as cyan at low levels (thick low level water clouds) and then towards green (mixed phase) and bright yellow at the cloud top (thick high level clouds). Again, this is due to an increasing presence of high level clouds, which strongly reflect in the red component, that is particularly sensitive to high level clouds at 1.3µm.